The present invention relates to a window for a light emitting diode formed of an aluminum-gallium-indium-phosphate (AlGaInP) compound.
A semiconductor light emitting diode (LED) includes a substrate, a light emitting structure, and a pair of electrical contacts for powering the diode, i.e., a substrate contact and a window contact. The substrate may be transparent or opaque; and the xe2x80x9csubstratexe2x80x9d contact, is formed on a surface thereof. The window contact is formed on a window surface.
LED structures composed of AlGaInP compounds can be designed to emit any selected one of a range of colors by tailoring the amount of Al in the compound. However, where the substrate and lower cladding layer are of n type AlGaInP compounds, it is difficult to achieve a low resistance p type AlGaInP compound for the upper cladding layer. Unfortunately, a relatively high resistance upper cladding layer does not provide full use of the surface of the light emitting structure. That is, current flowing between the window and substrate contacts tends to concentrate in a narrow xe2x80x9cfavoredxe2x80x9d path, which lies directly under the window contact. Thus, only that portion of the light emitting surface which lies in the favored path is activated.
A number of conventional arrangements provide a xe2x80x9cwindowxe2x80x9d which is interposed between the light emitting structure and the window contact to more fully utilize the light emitting surface. The conventional windows range from a single, thick layer of compounds other than AlGaInP, to a variety of multi-layer structures which xe2x80x9cspreadxe2x80x9d the energizing current across the face of the light emitting surfaces.
Light generated by an LED exits directly from the outer face of the light emitting surface or via the window. The xe2x80x9cwindowxe2x80x9d contact is formed on the outer face of the light emitting surface, or is part of the window.
In any event, the window contact covers a substantial surface area and blocks emission of light generated directly thereunder. For example, in the case of an LED having a 10 mil by 10 mil square window, a four mil round metal contact will obscure about 12.2% of the window surface. However, the window contact cannot be measurably reduced in diameter, since the contact must be large enough to ensure its adhesion to the window surface.
For example, FIG. 1A represents a top view of a conventional LED having a 10 mil by 10 mil square window with a top layer 105, the window having a 4 mil circular metal contact 106. Typically, the window contact is made of gold (Au). The conventional LED of FIG. 1B includes a metal substrate contact 101, an xe2x80x9cnxe2x80x9d GaAs substrate 102, an xe2x80x9cnxe2x80x9d cladding layer 103, an active region 104, a p cladding layer 105, and a metal window contact 106. As explained earlier herein, current which flows between the window contact 106 and the substrate contact 101 concentrates in a xe2x80x9cfavoredxe2x80x9d path directly under the window contact 101. Since, only a small area of the active layer lies in that current path, much of the light emitting potential of the LED is dormant. Additionally, most of the light emitted through layer 105 is intercepted by the opaque contact 106. In routine observations, it is noted that under the stated conditions, the light which is emitted by the LED appears as a thin halo surrounding contact 106.
Thus, an improved window structure which allows increased emission of light therethrough when used with AlGaNP compounds, is needed.
An apparatus consistent with the present invention includes a semiconductor light emitting diode having a substrate, a substrate electrical contact, a light emitting structure, and an improved window. The window interfaces directly with the light emitting structure, and, in the following stated order includes a lightly doped p GaP layer, a low resistance p GaAs layer, a transparent, amorphous conducting window layer, and a metal window contact. The conducting layer, by way of example, may be formed of indium tin oxide (ITO), tin oxide (TO), or zinc oxide (ZnO). Layers of other amorphous, conductive and transparent oxide compounds also may be suitable for construction of the window layer.
In a first embodiment consistent with the present invention, the metal contact passes through both the conducting layer and the GaAs layer to: (a) form an ohmic contact with those layers, and (b) contact the GaP layer and form a Shottky diode connection therewith.
In a second embodiment consistent with the present invention, the metal contact passes only through the conducting layer and contacts an insulator which is formed in the GaAs layer to isolate the metal contact from the GaP layer. As in the first embodiment, the metal window contact forms an ohmic contact with the conducting layer and the GaAs layer.
Advantageously, in both embodiments, the current path lying directly under the metal contact is eliminated and the current is widely spread over the face of the light emitting structure.
With elimination of the xe2x80x9cfavoredxe2x80x9d path, less light is generated from the portion of the diode interface which is directly under the window contact; and more light is generated from the remaining surface of that interface. The net result is an increase in the total light emitted through the window layer.
Advantageously, in accordance with this invention, all of the diode, other than the metal contacts, and the conducting layer, is grown in a continuous process.